What Is the D-Dimer Test?

The D-dimer test is a blood test doctors can use to rule out a severe blood clot. It's useful if your doctor suspects you have a blood clot in your lung or deep within a vein of your leg or pelvis.

These clots can be fatal. Getting prompt treatment increases your chances of surviving and avoiding other medical problems.

This article discusses the D-dimer test, how doctors use it, and its limitations. It also explains the link between COVID-19 and elevated D-dimer levels.

What Is D-Dimer?

D-dimer is a substance involved in the body's healing process. When you get an injury that causes you to bleed, your body uses proteins to clump up your blood. The clot that forms plugs the damaged vessel.

Once the bleeding stops, your body sends out other proteins to slowly break down the clot. Afterward, you end up with fragments of D-dimer in your blood.

These protein fragments usually dissolve over time. However, if a clot doesn't break up or another one forms, you'll have high levels of D-dimer in your blood.

Purpose of the Test

The D-dimer test involves a simple blood draw. A healthcare provider will use a thin needle to get a sample of your blood and analyze it. Results are ready within minutes.

As many as 100,000 deaths occur each year in the US due to DVT and PE.The symptoms you may have include:

• Swelling or redness, usually in the lower leg but sometimes in the thigh, pelvis, or an arm
• Pain in the leg, thigh, pelvis, or arm
• Difficulty breathing
• Fast heartbeat
• Chest pain
• Sweating a lot

Getting prompt treatment increases your chances of surviving PE and DVT. It also helps you avoid other medical problems that can hurt your quality of life.

When Is It Useful?

Diagnosing these conditions can be tricky. One study found that nearly 70% of people seen in outpatient clinics and emergency rooms with symptoms of a DVT didn’t have one.

Doctors used to have to send all blood samples to a central lab for analysis. This caused delays and meant the test couldn't be used for emergencies. So doctors were forced to send patients for expensive imaging tests instead.

Interpreting Results

Results can vary depending on the test your doctor used and the design. Doctors need to know the ranges of normal and abnormal levels for the test they're using.

If your results are in the lower range, your doctor can safely rule out a blood clot. If your results come back abnormal or high, you'll likely need more tests. The D-dimer test cannot be the sole basis to diagnose DVT or PE.

Why Is the D-Dimer High?

Many diseases, treatments, and lifestyle factors can raise your D-dimer levels. That's why it is essential to thoroughly answer your doctor's questions about your medical history. People with blood clots often have one or more of the same risk factors. They include:

Medical conditions and treatments:

• Heart disease: Patients with unstable angina or who have had a heart attack have higher levels of D-dimer and a higher risk of future blood clots.
• Cancer: Some cancers can increase the risk of a blood clot.
• Cancer treatment: Chemotherapy and certain breast cancer drugs can increase the risk of blood clots.
• Treatment with estrogen: Birth control pills and hormone replacement therapy can increase the risk of DVT and PE.
• Surgery: Patients who have had major surgery, like a hip or knee replacement, have a higher risk of a blood clot. (Drugs are prescribed to prevent this.)
• Infectious diseases: COVID-19 and pneumonia can cause inflammation and trigger blood clots.
• Kidney disease: For reasons that aren't fully understood, kidney disease increases the risk of DVT and PE.
• Liver cirrhosis: People with severe liver disease have a higher risk of clots in the large vein of the liver.
• Pregnancy: D-dimer levels rise two- to four-fold by delivery. Women have an increased risk of DVT or PE for up to three months after delivery.

Other risk factors:

• Age: People over 60 years of age have a higher risk of blood clots.
• Cigarette smoking
• Race: African Americans have higher levels of D-dimer compared to people of European ancestry.
• Gender: Women have higher levels of D-dimer than men.
• Obesity
• Sedentary lifestyle: Not exercising or not moving for an extended period can increase the risk of DVT or PE. An example is a long plane ride or being in the hospital.

Doctors will order other tests to make sure you don't have PE or DVT. These tests may include:

• Other blood tests: To see if you have a bleeding disorder
• Ultrasonography: A test that uses high-frequency sound waves to take pictures of your blood vessels, tissues, and organs
• Ventilation-perfusion lung scan: A test that uses a radioactive substance to help doctors see if air and blood can move through the lungs or if you have a blockage
• Computed tomography angiography: A test where you receive an infusion of a special dye. Doctors use a CT scan to take high-definition pictures from different angles. The dye lights up the blood vessels and tissues they need to check for blood clots.

Pulmonary Embolus

Patients with a low risk of blood clots and whose D-dimer levels are in the lower to middle range likely don't have a pulmonary embolism. Studies show the D-dimer test is comparable to ultrasonography or CT angiography in ruling out PE.

If your test results show high levels, you will need more tests. Also, many people who've had a recent PE will still have elevated D-dimer levels. So the test isn't helpful for them.

Deep Vein Thrombosis

Nearly all patients with DVT have elevated D-dimer levels. This makes the test beneficial in ruling out the condition for patients with levels in the lower to middle range. The test is also helpful if your symptoms aren't that clear. If your levels are high, your doctor will order more tests.  

Other Medical Conditions

Ruling out DVT and PE are the main reasons doctors order D-dimer tests. However, the test can help doctors evaluate and manage other severe conditions that involve blood clots. These include:

• Coronary artery disease: People with severe heart disease have higher levels of D-dimer. People who are treated for a heart attack but still have elevated D-dimer levels are at an increased risk of having another heart attack or dying from one.
• Stroke: Higher levels of D-dimer are associated with an increased risk of stroke.
• Disseminated intravascular coagulation (DIC): This is a rare disease in which blood clots form in vessels throughout the body. Elevated D-dimer levels are part of the scoring test for DIC.
• Hyperfibrinolysis: This blood clotting disorder is similar to DIC. The d-dimer test also helps doctors evaluate this disorder.

Summary

Doctors may order a D-dimer test if they suspect you might have a dangerous blood clot. The test helps doctors rule out two conditions that can be fatal: deep vein thrombosis, a blood clot in a vein, and pulmonary embolism, a blood clot in the lung.

A negative test result means you probably don't have a blood clot. Usually, you won't need any further tests. However, if your results come back high, that doesn't necessarily mean you have a clot. The test isn't definitive. Your doctor will likely order other tests.

A Word From Verywell

Doctors used to have to send all patients they suspected of having DVT or PE for imaging tests. This was expensive and time-consuming. The D-dimer test is a fast, inexpensive blood test they can use to rule out a dangerous blood clot. If your results come back low, you likely don't have to worry.

However, many diseases, treatments, and lifestyle factors can increase your D-dimer levels. So if your results come back abnormal, you'll need more tests to confirm you don't have a blood clot.